The present invention concerns processing of integrated circuits and pertains particularly to a laser interferometry endpoint detection with windowless polishing pad for chemical mechanical polishing process.
In a semiconductor manufacturing process, on a semiconductor wafer small electronic devices are formed of separate dies. The semiconductor wafer is processed using materials that are patterned, doped with impurities, or deposited in layers.
It is often necessary to polish a wafer surface to provide a substantially planar surface. This is done, for example, using a chemical-mechanical polishing process. Chemical-mechanical polishing is performed by pressing semiconductor wafer against a rotating polishing pad under controlled chemical, pressure, and temperature conditions. A chemical slurry, such as alumina or silica can be use as a polishing abrasive. The polishing effect on the wafer results in both a chemical and mechanical action.
In situ laser interferometry can be used to determine the end point of a chemical-mechanical polishing process. For example, an optical laser and optical radiation detector are located in a polishing platen. A transparent window is embedded into the platen surface for radiation transmission. The polishing pad has a matching embedded window made of a material that allows transmission of the laser radiation ("windowed pad"). The window embedded in the polishing pad is aligned to the window embedded on the platen so that radiation may be transmitted through the platen window and through the pad window. The aligned platen and pad windows can be referred to collectively as an "endpoint window.
As the platen rotates, the endpoint window encounters the wafer once per rotation, allowing radiation to be reflected from the wafer back through the window to the detector. During polishing of a transparent film that is coated over a substrate (e.g., silicon dioxide over silicon), as the film is removed from the surface, the intensity of the radiation at the detector has a periodicity governed by Equation 1 below: